This invention relates to a method of inhibiting the discoloration of emulsions due to contact with ferrous metals and ions of ferrous metals and compositions of emulsions which are resistant to such discoloration.
The inventors and others have noted that emulsions, and particularly, silicone containing emulsions, sometimes discolor, changing from their normal white color to yellow, brown or pinkish hues. This discoloration is considered to be very unsatisfactory by consumers of these products who value emulsions having a clean white color.
It was traditionally believed that this type of discoloration was due to a bloom, or increased growth, of microorganisms in the emulsion. To determine if microorganisms cause the discoloration, four samples of amino substituted polydimethyl siloxane oil in water emulsions were obtained from different sources for testing. Color observations and microorganism counts were made on the four samples as received. The results of the observations and counts are reported below.
TABLE 1 ______________________________________ SAMPLE COLOR ORGANISM COUNT ______________________________________ Sample A white 2.5 .times. 10.sup.6 cfu/ml Sample B very pink 3.1 .times. 10.sup.6 cfu/ml Sample C pink 1.4 .times. 10.sup.6 cfu/ml Sample D very pink &gt;3.0 .times. 10.sup.6 cfu/ml ______________________________________ cfu = colony forming units?
Testing indicated no apparent correlation between emulsion color and the number of microorganisms present in the samples. For example, Sample A was white in color and had a microorganism count nearly twice as high as that of pink hued Sample C.
Other inquiries were made which led to the postulation that the discoloration of the emulsion was due to contact with ferrous metals or ions of ferrous metals. Tests were conducted to compare the iron content of both white and discolored samples of silicone emulsions. A Milton Roy Spectro Kit for Soluble Iron Analysis, commercially available from Milton Roy Company, Rochester, N.Y., was used to determine soluble iron presence in emulsion samples. The analysis kit utilizes 1,10-phenanthroline as a colormetric indicator. It was found that discolored emulsion samples tested positive for soluble iron content while samples of white emulsions tested negative for soluble iron content.
In an experiment designed to intentionally discolor a white emulsion, a portion of Sample E was diluted with deionized water to obtain a 1% solids concentration of the emulsion. A coupon of 1018 mild steel, slightly smaller in size than a dime, was first rubbed clean with 4/0 coarseness emery paper, and then rinsed successively with 95% methanol then distilled water. The coupon was then placed in a closed bottle containing an amount of clean, white emulsion and the effect of the interaction between the ferrous metal and emulsion was observed over a two day period. At the end of the first day of contact, both the emulsion and steel coupon showed signs of deterioration. The emulsion had turned a yellow-brown color and a ring of red-brown precipitate, presumably an iron oxide, had formed in the bottom of the container around the coupon. At the end of two days of observation the emulsion had turned brown and the coupon itself had become very discolored from oxidation.
It was recognized that sources of iron which could cause discoloration of emulsions are numerous. Some possible sources of iron include the soluble iron found in the water used in formulating emulsion compositions or finished products, ferrous metals present in emulsion processing equipment and the containers in which the emulsions are transported and stored, especially the bung hole covers of drums or barrels.
A way was sought to prevent the discoloration of emulsions which come in contact with ferrous metals. It was discovered that the addition of phosphate ions, particularly amounts of one or more members of the group consisting of phosphoric acid and disodium phosphate to the emulsion prevented discoloration in the presence of ferrous metals and ions of ferrous metals.
It was found that the further addition of amounts of one or more members of the group consisting of zinc pyrithione and a resin comprising zinc oxide, ammonia and siloxane, acted to inhibit the pitting corrosion of ferrous materials often associated with phosphate contact at low pH levels. It was also found that the zinc pyrithione and zinc containing resin have the added benefit of controlling microbiological activity in emulsions.
U.S. Pat. No. 4,668,507 relates to additives for inhibiting pitting corrosion of metallic containers in which insecticide emulsions are sold. Disodium phosphate is disclosed as one additive component.